1. Field of the Invention
The present invention relates to a signal processing device for an endoscope which performs signal processing on an image-pickup device included in the endoscope.
2. Description of the Related Art
Electronic endoscopes including a solid image-pickup device at the distal portion of the inserting section of the endoscope have been widespread. For example, as disclosed in Japanese Patent Laid-Open No. 2001-29313, there has been proposed an electronic endoscope including a solid image-pickup device with an amplifying function inside the solid image-pickup device. As in the precedent example, an electronic endoscope with an amplifying function inside a solid image-pickup device can perform variable control on a signal level of an output signal that is outputted from the solid image-pickup device by applying an amplification ratio control signal for performing variable control on an amplification ratio (or sensitivity) from a signal processing device side, and therefore it has an advantage of obtaining an image with a good S/N even in a gleam such as in a fluorescent observation.
When a solid image-pickup device which can vary an amplification ratio by applying such an amplification ratio control signal is housed at the tip of an electronic endoscope, it is desirable to house the device by reducing it in size as small as possible as in the case of a usual solid image-pickup device.
For that purpose, as a charge coupled device (abbreviated as CCD) 97 which is included in a conventional electronic endoscope and can vary an amplification ratio, as shown in FIG. 15A, one with a narrowed horizontal width (number of pixels) in an optical black area (abbreviated as OB area) compared to a CCD98 (see FIG. 15B) (which does not need to be reduced in size as compared with the case where it is included in an endoscope) is adopted.
When a signal charge accumulated in an image area by using the CCD97 is read out via a horizontal transfer channel, it results in as shown in FIG. 15C, for example. As it is shown in FIG. 15C, the read out output signal may include a pixel which greatly deviates to higher level from a normal photoelectric conversion level, i.e., a defective pixel.
Such a defective pixel is usually called a white spot. The white spot occurs due to impurities in a photo diode being formed. The white spot exhibits characteristics which depend on temperature. The higher the temperature, the greater the white spot's influence is. Specifically, as shown in FIG. 15D, the intensity of a white spot (output level) increases almost in proportion to temperature.
The intensity of a white spot increases as an amplification ratio or an accumulation time increases. More specifically, as shown in FIG. 15E, the intensity increases almost in proportion to an amplification ratio.
As it has characteristics like that, it is desirable to reduce an influence of a white spot in a medical electronic endoscope which is inserted in a human body for endoscope examination and used in a state at a temperature higher than a room temperature. Although it is also conceivable that it is cooled by a Peltier device, the Peltier device causes the tip of the inserting section of the electronic endoscope to be thicker.
If an endoscope image is obtained by signal processing using the CCD97, the CCD output signal is inputted to a CDS circuit which performs correlation double sampling (abbreviated as CDS). The CCD output signal needs to be subject to analog clamping at the previous stage so as to be adjusted to an input range of the CDS circuit.
In the precedent example, if it is inputted into an analog signal processing circuit such as a CDS circuit or the like, it was clamped in an optical black (abbreviated as OB) area of the CCD97. Therefore, if a white spot is present in the OB area, it is clamped at a potential level higher than the potential level which originally needs to be clamped. That relatively decreases an output level of an image area and degradation of an image quality such as appearance of black lines in an image occurs (explained again in FIG. 6B as described later).